Portland cement may be prepared by sintering a mixture of components including calcium carbonate (as limestone), aluminum silicates (as clay or shale), silicon dioxide (as sand) and miscellaneous iron oxides. During the sintering process, chemical reactions take place wherein hardened nodules, commonly called clinker, are formed. Portland cement clinker is formed by the reaction of calcium oxide with acidic components to give primarily tricalcium silicate (designated by cement chemists as “C3S”), dicalcium silicate (“C2S”), tricalcium aluminate (“C3A”), and a ferrite solid solution phase in which tetracalcium aluminoferrite (“C4AF”) is present. The hydration of Portland cement with water is a complex process having different reactions among its primary components (C3S, C2S, C3A and C4AF). Some of these reactions occur at different times and may interfere with each other.
The production of Portland cement is energy intensive and releases carbon dioxide into the atmosphere. In an effort to reduce costs and carbon dioxide emissions, the concrete industry has increasingly used supplementary cementitious materials (“SCM”), such as fly ash. The production of SCM blended cements requires less energy and emits less carbon dioxide than Portland cement, because part of the Portland cement is replaced by SCM. Similarly, concrete produced with a blend of Portland cement and SCM embodies less energy and less production of CO2 than a concrete produced with Portland cement alone.
Thus, cements are increasingly produced having large amounts of SCM such as fly ash, which is a byproduct of coal manufacture. Fly ash is often used for blending with cements. It is able to contribute to the formation of the calcium silicate hydrates when blended with Portland cement. Preferred fly ashes for early strength development are those having high levels of calcium. In the United States, these fly ashes are classified as ASTM Class C fly ash.
The modern concrete industry continues to replace an increasing fraction of Portland cement with fly ash. The replacement level is approaching 30% and higher, because the growing concern is to reduce cost and carbon dioxide emission.
Unfortunately, the more reactive fly ashes, such as ASTM Class C fly ashes, are known sometimes to impact adversely the hydration of Portland cement, particularly, where the fly ash is used at levels around 20% or more. As these fly ashes are rich in calcium and aluminum, their use requires the sulfate ions supplied by Portland cement. As higher levels of Portland cement are replaced by Class C fly ash either in the cement or later in concrete or mortar production, the greater is the risk that the hydrating cement composition will become sulfate deficient. Class F fly ashes have a lower risk because their lower calcium content causes lower reaction rates.
While addition of sulfate materials (e.g., calcium sulfate) to blended cements having Class C fly ash can often restore hydration rates, thereby preventing abnormal set retardation and loss of early strength, such mixtures often do not respond favorably to chemical admixtures such as certain water reducing agents and non-chloride accelerating agents that are commonly used in the industry. A commonly found problem is an extended set retardation and slower than expected strength development. Set retardation and loss of early strength are undesirable because these generate delays and increase costs.
It is an objective of the present invention to provide novel methods for resolving the problems created by fly ashes, and in particular ASTM Class C fly ash and other calcium-containing fly ashes, such that use of such fly ashes in a blended cement or in the production of concrete or mortar does not substantially impair the desired hydration of the Portland cement fraction of the blended cement mixture.